In a large bowl, stir togeather the cookie mix and flour, using a pastry blender, cut in butter untill mixture resembles fine crumbs, then stir in the 1/2 cup of finely chopped macadamia nuts. Press the crumb mixture into the prepared pan, Bake in the pre- heated oven 12 to 15 minutes untill edges are lightly browned.The crust should still be hot when you pour in the filling. Leave oven on as set, you will be putting back in the oven after you pour in the filling .
For the Filling - In a medium Bowl, whisk the egg yolks, sweetened condensed milk, eggnog, rum OR rum extract, vanilla, and the 1/4 tesp. nutmeg, untill very well combined. Pour the mixture over the HOT Crust.
Bake for 15 to 20 minutes or untill the filling is set, Cool IN the pan on a wire rack for 1 hour . Then Cover and put in Refrigerator to completely cool, at least 1 hour ( note I would make mine the day before needed ) When cold, using the overlapped foil lift UNCUT bars out of panpull foil away from Bar Edges and cut into Bars .
If a Topping is Desired - In a Medium Bowl , Combine the whipping Cream and Powdered Sugar , Beating with an Electric Mixer on Medium, untill soft peaks form, ( when lifting up beaters, tips will curl ) also if desired, sprinkle on some finely chopped macadamia nuts, top with whipping cream and also if you like you could sprinkle on a little nutmeg. FOR STORAGE - Layer Bars between sheets of Waxed Paper in an Airtight Container; Cover Store up to 3 days OR you can Freeze them ( without any topping ) for up to 3 Months - These need to be Kept in the Refrigerator till ready to be served .